1. Field of the Invention
This invention relates in general to the use of femtosecond lasers to produce corneal cuts in the correction of astigmatism and more particularly to the generation of a nomogram for directing a femtosecond laser for making such cuts in conjunction with lens replacement surgery.
2. Description of the Related Art
Cataracts are areas of opacification of the lens of the eye of sufficient size to interfere with vision. They have been extensively studied because of their high prevalence in the elderly. Cataracts in the elderly (senile cataracts) are the most common type, and are often thought to be due to an acceleration of the scattering of light as it passes through the lens. Cataracts occur to varying extents in all humans over the age of 50, but generally do not cause significant visual problems until the ages of 60-80 years. In some instances, however, cataracts can occur much earlier as a result of risk factors including congenital disease, trauma, and family history.
Generally, correction of an occurrence of cataracts in an individual requires surgical intervention. In the surgical correction of the lens, an optical surgeon will make two relatively small incisions through the cornea of the eye at generally 90 degrees from each other. The major incision being on the outside of the cornea with the other being positioned based on the handedness of the surgeon. The surgeon will then remove the natural lens with the cataract, and replace the lens with an artificial lens. The incisions can then be sutured or otherwise closed, and the eye is allowed to heal.
Because cataract surgery involves replacement of the natural lens with an artificial lens, there is a side benefit to cataract surgery. Focal errors in the natural lens can be corrected through the correct selection of an artificial lens. Specifically, if a person was nearsighted (myopic) due to a focal length issue caused by the natural lens focusing light in the eye in front of the retina, the replacement lens can be selected to have a longer focal length than the natural lens that was removed. The person recovering from cataract surgery may have better vision than they did before getting cataracts as the focal point is now correctly on the retina. A similar choice of lens can be made to shorten the focal length if the person is farsighted (hyperopic). In effect, correction of cataracts can correct other problems with vision which may have nothing to do with the cataract itself.
As part of this additional vision correction, some patients would like to not just have better vision from cataract surgery, but to have good enough vision that they no longer need to wear corrective lenses. As surgical methods to improve sight, such as laser-assisted in situ keratomileusis, or LASIK, have become more common, the need for eye surgery (which is often a scary proposition) now often brings with it a desire to correct everything at once and patients who are undergoing eye surgery often desire to come out of it with perfect vision, regardless of the reason it was needed in the first place.
While a replacement lens can correct a number of focal problems from myopia or hyperopia, specifically, the lens can be chosen to be a lens which may have a different focal distance from the natural lens in order to correct focal imbalances between the natural lens and the cornea, it cannot correct all vision problems. Specifically, the replacement lens is essentially a perfect lens. Thus, patients who have focal problems that are not due to the focal length of their cornea and lens combination, but have focal problems due to focal imperfections in the cornea causing distortion, the lens replacement may not only fail to correct vision, but could make it worse. The most common visual impairment of this type is astigmatism, the condition where the lens is not symmetric but is squashed or bowed along a particular axis.
It is known that cutting incisions in the cornea which are then allowed to heal can result in alteration of the shape of the cornea and that such alterations can be beneficial in improving focal problems with the eye. The radial keratotomy procedure utilized this exact type of cuts to reshape the cornea and LASIK is essentially an improved technique on the same type of procedure. While this can be used to reshape the cornea to result in repositioning of the focal point (as is done in LASIK), it can also be used to correct for astigmatism by causing the cornea to heal in a fashion that regains a target shape from its squashed shape.
When it comes to astigmatism, the surgical procedure to replace a lens often induces a slight astigmatism (or may correct an existing astigmatism) by the very need to cut through the cornea to access the lens. This “induced” astigmatism, and the ability to provide additional correction at the same time, has led to astigmatism correction often being performed simultaneous to a lens replacement to help improve vision. While the correction can be performed via LASIK, such a major procedure is generally not needed and therefore astigmatism correction in conjunction with lens replacement is often a much simpler procedure.
Traditionally, the modification of the eye to reduce astigmatism is performed by performing one or more arcuate cuts towards the peripheral part of the cornea. These cuts are generally not all the way through the cornea, but only part way to allow for the cornea to alter its relative pressure and reshape without being fully penetrated. Obviously, correction of an astigmatism requires that the cuts be both correctly positioned (generally along the long axis of the astigmatism) and the correct length and depth to result in the correct reshaping from the more parabolic arc of an astigmatic eye, to the more hemispheric arc that is normal.
In order to know how to perform the cuts, doctors have relied for many years on nomograms, which are essentially cutting maps. These can be generated from hand calculations or from websites such as www.lricalculator.com where a doctor can enter the specifics of a patient's eye and be provided with the locations and size of cuts to be performed.
These websites and systems, however, suffer from a couple of major flaws. For one, the calculations are necessarily crude and lack detail. A human is simply incapable of cutting an arc of 24.6° consistently and therefore the nomograms tend to have a roughness with values provided in 5° arcs to take into account a surgeon's slop in simply not being able to make the correct sized cuts. Further, while cuts are supposed to be to a fixed depth, the depth of a cornea is often not known in a hand surgical procedure, and even if it is and a guarded blade of fixed penetration is used, the cut may not actually follow the prescribed depth at all points due to variations in the structure of the cornea.
Because of the inherent “sloppiness” in the making of such cuts by hand, while a traditional nomogram can provide useful guidance to a surgeon, it is, in many respects, no better than an educated guess and a skilled surgeon could often make a fairly accurate correction even without knowing what size cut to make at all. Further, it is difficult, if not impossible, to verify the accuracy of current nomograms as results on two patients with an identical procedure being performed may be different simply due to the skill, and luck, of the doctor performing the procedure.
Traditional nomograms are often based on simple mathematical relationships with a few centrally defined charts and relationships. In many nomograms, the charts provide for generally linear or other relatively simple mathematical relationships between variables in the various dimensions. Generally, there are three variables that matter in selecting how big of a cut to make. These variables are: 1) The axis of the astigmatism, 2) the amount of the astigmatism, and 3) the age of the patient. While the need to understand the first two variables should be obvious, the third is desired as the cornea changes in flexibility over life and thus older patients' corneas generally need smaller cuts than younger patients.
While the process of hand cutting is necessarily fraught with human error, recent advances in computer controlled cutting implements, such as ultrafast laser knives like the femtosecond laser, have provided tools where the surgeon's skill (and luck) with a blade are removed. The laser can cut precise cuts over and over again with a level of precision that the human hand is simply incapable of replicating. This type of tool, therefore, shows huge promise in being able to make vision correction much more accurate.
However, the tool is only as good as the skill of one using it and while a femtosecond laser may be able to make precise cuts, a surgeon still needs to know where to make those cuts. Nomograms for hand cutting are not useful for such surgery as their assumed degree of slop, and their course measurement, render more noise than value when the cutting instrument is accurate.